Fg125la

As mentioned previously, it is necessary to adjust the back-coupling efficiency, the working distance, and/or the spot size of the beam at the working distance of a LOF probe, depending on specific applications. These adjustments can be made by two parameters: the length of the beam expansion region (L_c), and the radius of curvature (R) of the fiber lens. In order to clearly see the effect of these two parameters, a total of 17 LOFs were fabricated with conventional SMF (Corning, New York, NY, USA, SMF-28) by an arc discharge of an optical fiber fusion splicer (FITEL, Tokyo, Japan, S183 PM) [9 (link)]. According to the fiber’s datasheet, the physical core diameter was 8.2 μm, and the MFD was 10.4 μm at a wavelength of 1550 nm. The refractive index of the CSF (Thorlabs, Newton, NJ, USA, FG125LA) was 1.444 at the same wavelength. Based on the simulation, the fabrication was intended to provide the same working distance, but different back-coupling efficiencies.

The sensor is fabricated by splicing a section of coreless (CL) fiber FG125LA (Thorlabs) to a single-mode fiber (SMF-28) pigtail, see Figure 1. The difference in numerical aperture between the two fibers enables the excitation of multiple modes in the CL fiber, which create the interference spectrum. A silver mirror is chemically deposited at the end-face through the mirror reaction. The reactant solution was prepared according to [25 (link)] and the tip of the fiber was placed in contact with the liquid. The fiber was withdrawn carefully without breaking the meniscus created by surface tension. This yielded a silver layer of an approximate thickness of 30 µm, and with minimal deposition on the side of the fiber. With a 2 × 1 coupler (Huber Suhner), a broadband source (Fyla, SCT500), and a grating-based spectrometer with 0.3 nm resolution (Ibsen I-MON 512), the multi-mode interferometer is interrogated. The length of the SMF between the source, detector and sensor, ensures that only the fundamental mode of the fiber can be assumed to be transmitted. Therefore, only the region from the splice to the end-face of the fiber is considered in the model.

The non-grated fiber tests were performed with an ordinary singlemode optical fiber (SMF-28; Corning Incorporated, Corning, NY, USA) that was terminated with a 10 cm coreless termination fiber (FG125LA; ThorLabs, Newton, NJ, USA) and freely suspended in a furnace (HF TAP 1210B; Evenheat Kiln, Inc., Caseville, MI, USA) during heating to temperatures of 100, 350, 550, 750, and 950 °C. The temperature was held for 40 min at each step and monitored using a Type K thermocouple. The grated fiber test was performed using a pure silica core, F-doped silica cladding singlemode optical fiber inscribed with 12 femtosecond Type-II FBGs (FemtoFiberTec, Goslar, Germany) centered near 1550 nm with a reflectivity less than 10%. This fiber was freely suspended in the same furnace and heated to 1130 °C with steps programmed to reach 350, 550, 750, 950, and 1150 °C, each held for 40 min.